While most DNA exists in the canonical B-DNA form, repetitive DNA sequences can form alternatively structured DNA (or non-B-DNA) such as H-DNA and Z-DNA. These non-B DNA-forming sequences are abundant in mammalian genomes and are often associated with diseases such as Fragile X Syndrome, Huntington's disease and myotonic dystrophy. We have found that H-DNA and Z-DNA-forming sequences are highly mutagenic in mammalian cells and in mice, and often co-localize with chromosome breakage hotspots, such as in the promoter region of the c-MYC gene that maps to a translocation breakpoint hotspot in Burkitt's lymphoma. Furthermore, the mutagenic H-DNA and Z-DNA-forming sequences stimulate DNA double-strand breaks (DSBs), leading to deletions and translocations, implicating them in translocation-related disease etiology. In addition to their intrinsic instability, it has been demonstrated that DNA damage from exogenous sources may be enriched in non-B-forming sequences and this damage may be refractory to repair. UV irradiation from the sun is a ubiquitous environmental carcinogen that we are exposed to on a daily basis and has been associated with cancers such as melanoma. In B-DNA, UV irradiation causes the formation of DNA lesions such as 6-4 photoproducts (6-4PPs) and cyclobutane pyrimidine dimers (CPDs). However, how exogenous DNA damaging agents such as UV irradiation affect H-DNA and Z-DNA structure formation, stability, and mutagenic potential is not well understood. Our long-term objectives are to understand how UV irradiation affects non-B DNA structure-induced genetic instability in human disease. The immediate objectives of this proposal are to test the hypothesis that non-B DNA regions will be more prone to UV irradiation damage, and more refractory to repair than B-DNA. We will test this hypothesis using a mammalian cell system and a mutation reporter assay our lab has previously developed. Specific Aim 1 addresses the mutagenic potential of non-B DNA sequences after UV exposure. The mutagenic potential of non-B DNA after UV irradiation will be tested in mammalian COS-7 cells using our mutation reporter assay. Specific Aim 2 addresses the potential of UV lesion formation within and surrounding non-B DNA-forming sequences and structures. We will determine how non-B DNA structures affect lesion formation and how lesion formation affects the formation of non-B DNA structures. Specific Aim 3 characterizes the repair pathways involved in processing UV lesions at the non-B DNA structures using cell lines deficient in different repair proteins.